(1) Field of the Invention
The invention relates to the fabrication of integrated circuit devices, and more particularly, to a new method of forming a dual damascene structure.
(2) Description of the Prior Art
One of the more important aspects of creating integrated circuit devices is the creation of metal interconnects that provide paths of electrical connectivity between the various components of the device. In view of the ever continuing decrease in device feature dimensions, these metal interconnects become relatively more important and can have profound impacts on the overall device performance. It is, for instance, highly desirable to achieve high wiring and packaging densities which brings with it the necessity to fabricate a multilayer structure on the substrate to connect integrated circuits to one another. These multi layer structures are separated by dielectric or insulating layers such as a polyimide. Via holes are created between adjacent layers of wiring that provide electrical connections between signal lines or to the metal power and ground planes. The design of the interconnect wiring network and the connecting points between these networks must adhere to strict design rules in order to maintain or enhance device performance while reducing device size and increasing device density. It is, for instance important to maintain good planarity between adjacent layers since lack of good planarity will have an increasingly negative effect on the planarity of overlying layers. Also, adjacent layers are ordinarily formed so that the primary signal propagation directions are orthogonal to each other. One of the design approaches that has been used for some time to create interconnect metal is the damascene and its extension the dual damascene process.
With the damascene process a metal via plug is first formed in a surface, typically the surface of a semi-conductor substrate. A layer of dielectric (for instance SiO2) is deposited over the surface (using for instance PECVD technology); trenches (for metal lines) are formed in the dielectric (using for instance RIE technology). Metal is deposited to fill the trenches; the excess metal on the surface is removed. A planar structure of interconnect lines with metal inlays in the (intra-level) dielectric is achieved in this manner.
An extension of the damascene process is the dual damascene process whereby an insulating or dielectric material, such as silicon oxide, is patterned with several thousand openings for the conductive lines and vias, which are filled at the same time with metal. Damascene is an interconnection fabrication process in which grooves are formed in an insulating layer and filled with metal to form the conductive lines. Dual damascene is a multi-level interconnection process in which, in-addition to forming the grooves of single damascene, conductive via openings also are formed. One of the dual damascene approaches uses a dielectric layer that is formed by three consecutive depositions whereby the central layer functions as an etch stop layer. This etch stop layer can be SiN, the top and bottom layer of this three layer configuration can be SiO2. This triple layer dielectric allows first forming the vias by resist patterning the vias and etching through the three layers of dielectric. The conductive pattern can then be formed in the top layer of dielectric whereby the central layer of SiN forms the stop layer for the etch of the conducting pattern. Another approach, still using the three-layer dielectric formed on the substrate surface, is to first form the pattern for the conducting lines in the top layer of the dielectric whereby the SiN layer again serves as etch stop. The vias can then be formed by aligning the via pattern with the pattern of the conducting lines and patterning and etching the vias through the etch stop layer of SiN and the first layer of dielectric. Yet another approach is to deposit the three layer dielectric in two steps, first depositing the first layer of SiO2 and the etch stop layer of SiN. At this point the via pattern can be exposed and etched. The top layer of SiO2 dielectric is then deposited; the conducting lines are now patterned and etched. The SiN layer will stop the etching except where the via openings have already been etched.
Yet another approach to forming the dual damascene structure is to form an insulating layer that is coated with a photoresist. The photoresist is exposed through a first mask with image pattern of the via openings, this via pattern is anisotropically etched in the upper half of the insulating layer. The photoresist now is exposed through a second mask with an image pattern of the conductive line. The pattern of the conducting lines is aligned with the pattern of the vias thereby encompassing the via openings. In anisotropically etching the openings for the conductive lines in the upper half of the insulating material, the via openings already present in the upper half are simultaneously etched and replicated in the lower half of the insulating material.
With the dual damascene process, the conductive line pattern and the vias are filled with metal at the same time which represents an improvement over the damascene process in that the dual damascene process has fewer processing steps.
In a typical dual damascene process, the dual damascene structure can be formed by etching the vias after which the trenches are etched. After the vias have been etched, a stop layer of for instance SiN or SiON can be deposited to serve as etch stop for the etching of the trenches. The disadvantage of the deposition of the stop layer is that SiN or SiON have a high dielectric constant resulting in an increase in the dielectric constant of the dual damascene structure. This is contrary to the design requirement of providing dielectric layers between conducting metal that have a dielectric constant that is as low as possible, ideally the dielectric constant of a vacuum. Another approach that has been highlighted above in creating the dual damascene structure is to etch the trench prior to creating the vias. The disadvantage of this approach is that the Depth of Focus (DOF) in creating the vias is too high for accurate photolithographic exposure of the vias.
The invention addresses these problems by teaching a method for creating a dual damascene structure that does not result in an increase of the dielectric constant of the intra-level dielectric and without having a negative impact on the contour definition of the via etch of this structure.
FIG. 1 provides an overview of the Prior Art method of creating a dual damascene structure. FIG. 1 shows a cross section of the opening that is etched through the layer 25 of intra-level dielectric. This intra-level dielectric 25 is deposited on a surface 20, typically the surface of a substrate. It is the objective of the dual damascene structure to make electrical contact with a point 12 of metal, for instance aluminum, in the surface 20 over which it is created. A layer 15 of photoresist has been deposited and patterned (to the width of the to be created via opening) on the surface of the dielectric 25.
FIG. 2 shows a cross section whereby a second layer 30 of photoresist has been deposited and patterned on the surface of the intra-level layer of dielectric 25. The width of the pattern that is created in the photoresist is equal to the width of the trench that needs to be etched in the upper region of the intra-level dielectric.
FIG. 3 shows a cross section whereby the pattern in the photoresist has been etched into the upper regions of the dielectric region of the layer 25. The width of this trench 14 is equal to the width of the pattern created in the second layer of photoresist. The width of the via 16 is the width of the opening etched in the first layer of photoresist. The depth of the trench can be controlled by means of a stop layer or by time control of the etch for the trench or by etch parameters such as the type of etchant that is used for opening the trench.
U.S. Pat. No. 5,792,823 (Avanzino et al.), U.S. Pat. No. 5,614,765 (Avanzino et al.) and U.S. Pat. No. 5,705,430 (Avanzino et al.) show dual damascene etch process with 2 IMD layers.
U.S. Pat. No. 5,882,996 (Dai), U.S. Pat. No. 5,877,076 (Dai), U.S. Pat. No. 5,686,354 (Avanzino et al.) and U.S. Pat. No. 5,635,423 (Huang et al.) show dual damascene process with 2 IMD layers and etch stop layers.
A principle objective of the invention is to create a dual damascene structure without increasing the dielectric constant of the dielectric between the interconnect line pattern.
Another objective of the invention is to provide a method of forming a dual damascene structure that does not use a stop layer for the trench etch while maintaining good trench etch end-point detection.
Yet another objective of the invention is to provide a method that allows for etching the trench of the dual damascene structure with high selectivity.
In accordance with the objectives of the invention a new method is provided for creating a dual damascene structure. Two layers of dielectric are deposited in sequence. The lower dielectric is the via dielectric and is selected such that it has a low etching rate and results in different volatile gas during the etch of the via. A first photoresist is patterned for the via, the etch for the via etches through both layers of dielectric. A second layer of photoresist is patterned for the trench etch, due to the difference in etch rate between the two layers of dielectric, the trench of the dual damascene structure is etched without further affecting the via etch in the lower layer of dielectric.